The present invention relates to screen printing machines in general, and more particularly to improvements in screen printing machines of the type where in a tubular stencil surrounds a rotary squeegee and is mounted in two spaced apart bearing members or heads at least one of which is adjustable relative to the frame of the screen printing machine.
The commonly owned Swiss Pat. No. 546,145 discloses a screen printing machine wherein each bearing member is adjustable in several directions so as to insure that the stencil can apply ink or another liquid to predetermined portions of a web of fabric which travels below the stencil along the upper side of a platform or table. In order to insure that the stencil will rotate at a desired speed and to simultaneously avoid undesirable twisting of the stencil, the screen printing machine normally comprises two discrete drives, one for each end portion of the stencil. This contributes to the initial and maintenance cost of the machine. It is further known to subject the stencil to substantial tensional (axial) stresses and to provide a drive which transmits torque to one end of the tensioned stencil. The tensioning is intended to prevent twisting of the stencil in view of the fact that only one end of the stencil is positively driven. Such proposals have met with limited success since the stencil cannot be tensioned at will because its material is incapable of standing substantial axial stresses. Therefore, the permissible tensioning of the stencil is insufficient to prevent the development of pronounced torsional stresses and the resulting twisting of the stencil when the drive means merely transmits torque to one end of the stencil. In other words, the tensioning of a stencil merely reduces but does not eliminate the danger of twisting when the drive means is designed to transmit torque to one end of the stencil.
Additional problems arise when the end portions of the stencil are mounted in adjustable bearing members, i.e., in bearing members which are movable relative to the frame of the screen printing machine. As a rule, the bearing members support rotatable driving sleeves which are coupled to the respective end portions of the stencil and each of which receives torque from a discrete bevel gear transmission. The input members of the transmission receive motion from the main prime mover, preferably through the medium of a roller which drives the back cloth for the fabric. At least one of the bearing members is pivotable about an axis which is normal to the axis of the stencil to thus enable the attendants to eliminate misalignment of imprints which are applied to the fabric at successive printing stations. Since the aforementioned driving sleeves are rotated by gear trains, any pivoting of a bearing member in the frame will produce substantial twisting stresses which tend to deform the stencil as soon as the pivotable bearing member is caused to leave its neutral position. The likelihood of twisting of the stencil is especially pronounced when the machine comprises two discrete drives, one for each end of the stencil. Thus, when one of the bearing members is pivoted from its neutral position, the gear which rotates the driving sleeve at the other end of the stencil acts not unlike a fixedly mounted toothed rack and causes the repsective driving sleeve to turn together with the respective end portion of the stencil. This can cause a total destruction or extensive deformation of the stencil. Attempts to prevent such undesirable twisting include the provision of manually operated means for rotating the gears at that end of the stencil which is supported by the pivotable bearing member. However, this causes the stencil to change its position relative to the fabric so that the pivoting of one bearing member for the purpose of eliminating one defect in alignment entails a different type of misalignment between the stencil and the fabric.